According to the National Electric Code (NEC), a plenum is a “compartment or chamber to which one or more air ducts are connected and [which] forms part of the air distribution system.” The concern is that during a fire, if there is burning material in a plenum air space, smoke and fumes can travel through air ducts to the whole building. For this reason, there are codes to restrict the types of materials (such as wiring) that can be placed in the plenum.
Currently, communications connections made in plenum space are often made via a surface mount box. A surface mount box is an open sided box that is mounted on the floor, walls or ceiling of a plenum space. The surface mount box is typically hollow and includes ports for establishing connections between communications cables, conductors and the like. The open end of the box is mounted to a surface such that the walls of the box protect the connections contained inside.
Surface mount boxes are regulated by UL 2043, which governs heat release and smoke of a box that catches fire. To date, boxes rated for plenum space have been formed of metal in order to pass UL 2043. However, the metal surface mount boxes can be rather expensive to manufacture and can render installation of the boxes and connection of cables difficult.